This might sound a little odd, but a surprisingly large part of why I main a Death Knight is for the extra 20% riding speed from On a Pale Horse.
Hopefully some of you understand the feeling: once you experience 492% flying speed, you can hardly go back to the measly 451% on any other character.

Paladins, of course, have the same mounted speed increase, but for many of the same reasons that have been mentioned in this thread I prefer DK greatly over Paladin. The main reason is that 2H Frost's rotation is probably the most engaging DPS rotation I've played with. I know that this is a common source of debate (and this is certainly not the thread to debate it!), but at the very least I think we could all agree that there is far more subtlety to it than simple Obliterate spam!

Also we have some pretty cool transmogrification options even though we have no sets from Vanilla or The Burning Crusade.

I liked how this Class was different from Others with different Mechanics, like a Ghould that you empower further to become an Abomination, or how instead of Abilities that increase your Movespeed you DeathGrip your Opponents to you etc.
Now the Classes are more washed up and Every Class can increase their own movespeed which is lame. Also every Class can summon something (except Rogue?)

I started playing at the tail end of BC as an alliance warrior. When Wrath released I joined my friend on his server and went horde. I didn't feel like starting a new toon at level 1 so I took the easy way out and made a dk so I could start at 55 and catch up him quicker. It's been my main ever since and I think he always will be lol. I love the variety the class offers, like the ability to solo so much old content. Sometimes it's just nice to log on and go do something by myself when I've become bored with doing the same current content for weeks and months on end. I have alts but my dk will always be my first love.

Mainly because I love the character from an RP perspective-- he's my favorite character to play ever.

Blood's fun mechanically, and my favorite tank class-- if only pugs didn't want me to throw my laptop out the window. I don't love either DPS spec-- unholy clashes with my character's RP so badly that the spec just feels wrong, but frost is a little /too/ simple to play-- it could use a slightly trickier rotation to be more interesting.

It's fun to actually feel like a.. well.. knight of death. You feel like you can make a difference in a fight by using a small army of minions. You get in there and beat people down and when they try to run you pull them back or slam them with frost filled wind and they go down. You feel like a Soul Reaper. :P

I love the Lore on them. Ever since Gul'dan raised them in WarCraft 2 and all until now, Ner'zhul, the Lich King is probably my favorite character and Arthas close second. Kel'Thuzad always felt like a true mentor in WarCraft 3 so he grew on me like a character that could lead me in battles. I also love the Necromancy art and I'm the kind of person who likes to go with the one that stands out, is not "normal".

Beside the lore, I just love the game mechanics, I'm a Plate user with a big weapon, raising minions, like a Necromancer. It's like a perfect class made for me since I love Necromancers and heavy plate wearing Warriors and I'm never sure what to go with in other games, but WarCraft since they made Death Knights <3